186 VLSI Test Principles and Architectures TABLE 4.6 ± PODEM Objectives and Decisions for g Stuck-At-1 getObjective() backtrace() logicSim() D-frontier g =0 a =0 g = D , c =0 d = 0, i =0 h (but no X-path to PO) a = 1 (reversal) — c = 1, d = 1 ∅ searches can be avoided. For instance, in this example, if the next objective was f = 1, even after the decision of b = 1 is made, the target fault still would not have been detected, as there was no path to propagate the fault-effect to a primary output even before the decision b = 1 was made. In other words, the search could immediately backtrack on the first decision a = 0. In this case, a = 1, and the objective is still g = 0. Backtrace() will now return b = 0. After logic simulation, g = 0, c = 1, f = 0, h = 0, i = 0. Again, there is no propagation path possible. As there is no earlier decision to backtrack to, the ATPG concludes that fault g/ 1is untestable. Table 4.6 shows the steps for this example. 4.4.5
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